1. Field of the Invention
The invention relates to a photomask based on which a pattern is defined, used in photolithography.
2. Description of the Related Art
A photomask is grouped into an emulsion mask including a photographic dry plate having a high resolution and a hard mask including a glass substrate and a patterned light-impermeable or thin metal film formed on the glass substrate. Though the hard mask is expensive relative to the emulsion mask, the hard mask is superior to the emulsion mask with respect to an ability of accomplishing a small line and a mechanical strength. Hence, the hard mask is predominantly used as a photomask in fabrication of a semiconductor device.
Presently, photolithography in which a so-called reticle, a hard mask having a pattern about five times greater in dimension than an original pattern, is used for step and repeat exposure is predominantly used in fabrication of a semiconductor device with respect to an ability of mass-production and readiness in fabricating a mask.
A pattern in a reticle used in photolithography is fabricated almost by electron beam painting. Hereinbelow is explained a conventional reticle and a conventional method of fabricating a reticle, with reference to FIGS. 1A, 1B and 2A to 2E.
FIG. 1A is a plan view of a conventional reticle 4, and FIG. 1B is a cross-sectional view taken along the line 1Bxe2x80x941B in FIG. 1A.
As illustrated in FIG. 1B, the conventional reticle 4 is comprised of a glass substrate 51 and a light-impermeable film 52 formed on the glass substrate 51. The light-impermeable film 52 has a pattern comprised of a chip pattern 41 and a frame pattern 42 formed around the chip pattern 41.
The chip pattern 41 is a mask pattern used for fabricating a gate electrode of a semiconductor chip and other parts, and is fabricated by forming openings in the light-impermeable film 52 in selected areas.
The glass substrate 51 is almost entirely covered with the light-impermeable film 52 in the frame pattern 42.
Around a periphery of the chip pattern 41 is formed an external frame line 43 as an opening of the light-impermeable film 52.
When a resist is to be exposed to a light through the reticle 4, at first, an object (not illustrated) to be etched, on which a resist has been coated, is mounted on an X-Y table equipped in an exposure unit (not illustrated), and the reticle 4 is mounted on a reticle stage (not illustrated) positioned above the X-Y table. After requisite alignment has been made, a light emitted from a light source is radiated to both the external frame line 43 and the chip pattern 41 through a condensing lens. The light having been radiated to the chip pattern 41 and the external frame line 43 passes through the reticle 4. The light having passed through the reticle 4 is condensed by another condensing lens, and then, projected onto the resist coated on the object to be etched.
Hereinbelow, a method of fabricating the reticle 4 is explained with reference to FIGS. 2A to 2E. FIGS. 2A to 2E are cross-sectional views of the reticle 4, taken along the line 1Bxe2x80x941B in FIG. 1A, illustrating respective steps of a method of fabricating the reticle 4.
In FIGS. 2A to 2E, a broken line 55 indicates a boundary between the chip pattern 41 and the frame pattern 42. An area located at the left of the boundary line 55 is the chip pattern 41, and an area located at the right of the boundary line 55 is the frame pattern 42. As illustrated in FIG. 2A, it is assumed that the chip pattern 41 is comprised of a peripheral portion 41a and a central portion 41b surrounded by the peripheral portion 41a. An area indicated with xe2x80x9cCxe2x80x9d indicates an area extending in the vicinity of the boundary line 55, including the peripheral portion 41a. 
As illustrated in FIG. 2A, there is first prepared a blank comprised of a glass substrate 51 and a light-impermeable film 52 formed on the glass substrate 51 by sputtering. The glass substrate 51 is composed of soda-lime glass, heat resistance glass or synthesized quartz, for instance. The light-impermeable film 52 is comprised of a thin metal film such as a chromium film. The light-impermeable film 52 may be comprised of a silicide film such as a molybdenum silicon compound (MoSi2) film.
Then, a positive resist 53 is applied onto the light-impermeable film 52, as illustrated in FIG. 2A.
Then, electron beams are radiated to the positive resist 53 in predetermined areas thereof. As illustrated in FIG. 2B, the electron beams are radiated to the resist 53 in areas a1, a2 and a3, and the electron beams are not radiated to the resist 53 in areas b1, b2, b3 and b4.
In place of the electron beams, ion beams may be radiated to the resist 53.
Then, the resist 53 having been exposed to the electron beams is soaked into a developing agent for developing the resist 53. By soaking the resist 53 into a developing agent, the areas a1, a2 and a3 are dissolved, and hence, removed. As a result, there are formed openings c1, c2 and c3, as illustrated in FIG. 2C.
The areas b1, b2, b3 and b4 are not dissolved, that is, remain as they are, and define resist patterns d1, d2, d3 and d4.
Then, the light-impermeable film 52 is dry-etched with the resist patterns d1, d2, d3 and d4 being used as a mask. As an etching gas, there is used a mixture gas containing Cl2 at 75 volume % and O2 at 25 volume %.
By carrying out dry etching, portions of the light-impermeable film 52 exposed through the openings c1, c2 and c3 are removed. A portion of the light-impermeable film 52 exposed through the opening c1 is removed to thereby define the external frame line 43, as illustrated in FIG. 2D. Portions of the light-impermeable film 52 exposed through the openings c2 and c3 are removed to thereby define the chip pattern 41, as illustrated in FIG. 2D.
Then, the resist 53 is removed. By removing the resist 53, portions of the light-impermeable film 52 covered with the resist patterns d1, d2, d3 and d4 appear to define light-impermeable film patterns e1, e2, e3 and e4, as illustrated in FIG. 2E.
Thus, the reticle 4 is completed.
The above-mentioned conventional reticle 4 is accompanied with a problem that a numerical aperture in the central portion 41b of the chip pattern 41 is higher than a numerical aperture in the area C. In other words, a darkness rate in the central portion 41b is lower than a darkness rate in the area C. This is because the area C includes the peripheral portion 41a and the frame pattern 42 having almost no openings, whereas a circuit pattern is formed entirely in the central portion 41b, and hence, the central portion 41b has many openings.
For instance, a gate pattern of a logic circuit has a darkness rate of 30% or smaller, that is, a numerical aperture of 70% or greater, resulting in a remarkable difference in a numerical aperture between the area C and the central portion 41b of the chip pattern 41.
Such a difference in a numerical aperture between the area C and the central portion 41b causes unbalanced etching such as partially excessive etching or partial shortage in etching in accordance with a numerical aperture due to local loading effect.
If such unbalanced etching is caused not in a depth-wise direction of an object to be etched, but in a plane-wise direction of the same, etching rates would become different in portions of an object to be etched, in accordance with numerical apertures in a plane-wise direction of the object, even if a resist is exposed to a light in accordance with a properly designed pattern.
This results in a problem that an accurate mask pattern designed in accordance with a circuit pattern cannot be obtained. In other words, the above-mentioned difference in a numerical aperture causes reduction in an accuracy with which a resist is etched for forming a desired pattern. Even if a photomask having a reduced accuracy with which a resist is etched is used, since images made by such a photomask are projected onto a resist, it would be impossible to form a resist pattern with high accuracy in accordance with a circuit pattern.
In the above-mentioned conventional reticle 4, reduction in an accuracy in a pattern is caused as follows.
As a result of reaction between an etching gas and an object to be etched, composed of chromium (Cr), there is produced oxygen gas (O2). The reaction formula is as follows.
2Cr2O3+4Clxe2x86x924CrCl+3O2↑
The reaction defined with the reaction formula produces oxygen gas (O2) in a relatively great amount in the central portion 41b having a relatively high numerical aperture, and oxygen gas (O2) in a relatively small amount in the area C having a relatively small numerical aperture.
Oxygen gas (O2) has a characteristic of etching the resist 53. Accordingly, oxygen gas (O2) as the product resulted from the above-mentioned reaction etches, and hence, narrows the resist patterns d1, d2, d3 and d4. Due to a difference in generation of oxygen gas (O2), the resist pattern 4d located in the central portion 41b of the chip pattern 41 is etched to a greater degree, and hence, becomes narrower than the resist pattern d3 located in the peripheral portion 41a. 
A portion of the light-impermeable film 52 which appears as a result of etching the resist patterns d1, d2, d3 and d4 is removed by etching. Accordingly, a volume of the light-impermeable film 52 to be etched in more than an area of the resist openings at the start of etching is greater in the central portion 41b than in the peripheral portion 41a. 
As a result, reduction in a width of the light-impermeable film pattern e4 located in the central portion 41b, relative to a designed width of the same, becomes greater than reduction in a width of the light-impermeable film pattern e3 located in the peripheral portion 41a, relative to a designed width of the same. This means that it is impossible to obtain a mask pattern in accordance with a circuit pattern.
Again, the conventional reticle 4 is accompanied with the problem that a difference in a numerical aperture between areas of the reticle 4 causes reduction in an accuracy with which a resist is etched for forming a pattern, as mentioned above.
In view of the above-mentioned problem in the conventional reticle, it is an object of the present invention to provide a photomask which is capable of eliminating unbalance in a numerical aperture between the peripheral portion and the central portion, thereby avoiding reduction in an accuracy with which a resist is etched for forming a pattern.
In one aspect of the present invention, there is provided a photomask including (a) a photomask substrate, and (b) a light-impermeable film formed on the photomask substrate, the light-impermeable film having a pattern comprised of a first pattern corresponding to a circuit pattern of a semiconductor integrated circuit, and a second pattern formed around the first pattern for adjusting a numerical aperture of the first pattern.
In accordance with the photomask, by adjusting a numerical aperture of the second pattern, it would be possible to increase a numerical aperture of the above-mentioned area C, and thereby equalize a numerical aperture of the area C to a numerical aperture of the above-mentioned central portion 41b. Hence, by balancing numerical apertures of the area C and the central portion 41b to such a degree that the chip pattern is adversely affected by local loading effect, it would be possible to avoid reduction in an accuracy with which a resist is etched for forming a pattern, and have a mask pattern or a resist pattern with high accuracy in accordance with a circuit pattern.
In a preferred embodiment, the second pattern is formed around the first pattern in an area to which a light is not radiated.
In the embodiment, a light is not radiated to the second pattern. Hence, it is possible to radiate a light only to the first pattern or chip pattern without radiating a light to the second pattern. As a result, the second pattern is not reflected to a pattern, ensuring that design data used for designing the second pattern optimal for increasing an accuracy in a chip pattern can be made without any bars.
In a preferred embodiment, the second pattern has a numerical aperture almost equal to a numerical aperture of the first pattern.
In accordance with the embodiment, it is possible to balance between numerical apertures in the area C and the central portion to such a degree that a chip pattern is not adversely affected by the local loading effect. Thus, it is possible to avoid reduction in an accuracy with which a resist is etched for forming a pattern, and have a mask pattern or a resist pattern with high accuracy in accordance with a circuit pattern.
In a preferred embodiment, the second pattern has a numerical aperture almost equal to an average of a numerical aperture in the entirety of the first pattern.
In accordance with the embodiment, it is possible to readily fabricate data for designing the second pattern which data is optimal for enhancing an accuracy of a chip pattern by using data of the chip pattern. It is also possible to balance between numerical apertures in the area C and the central portion to such a degree that a chip pattern is not adversely affected by local loading effect. Thus, it is possible to avoid reduction in an accuracy with which a resist is etched for forming a pattern, and have a mask pattern or a resist pattern with high accuracy in accordance with a circuit pattern.
In a preferred embodiment, the second pattern has a width equal to or greater than 10 mm.
The results of the experiments having been conducted by the inventor show that it is necessary for the second pattern to have a width equal to or greater than 10 mm in order to prevent the chip pattern from being adversely affected by local loading- effect. This is because the local loading effect could be found within 10 mm around the chip pattern in the conventional reticle 4. Accordingly, if the second pattern had a width smaller than 10 mm, it would be impossible to prevent reduction in an accuracy with which the chip pattern is formed.
In a preferred embodiment, the light-impermeable film has an almost uniform density in the second pattern.
In accordance with the embodiment, since the light-impermeable film has an almost uniform density in the second pattern, the second pattern does not cause unbalanced numerical aperture, and can properly adjust a numerical aperture of the first pattern. This ensures prevention of reduction in a pattern accuracy which reduction is caused by the local loading effect.
In a preferred embodiment, the light-impermeable film constituting the second pattern is equally spaced away from adjacent ones in both a first direction and a second direction perpendicular to the first direction.
In accordance with the embodiment, a pattern could be readily designed. In addition, the second pattern does not cause unbalanced numerical aperture, and can properly adjust a numerical aperture of the first pattern. This ensures prevention of reduction in a pattern accuracy which reduction is caused by the local loading effect.
It should be noted that a space between the adjacent light-impermeable films in the first direction is not always equal to a space between the adjacent light-impermeable films in the second direction.
In a preferred embodiment, the second pattern has a numerical aperture of 100%.
In accordance with the embodiment, since the light-impermeable film does not exist on the second pattern, it is no longer necessary to have the light-impermeable film have a uniform density on the second pattern. In addition, the second pattern does not cause unbalanced numerical aperture, and can properly adjust a numerical aperture of the first pattern. This ensures prevention of reduction in a pattern accuracy which reduction is caused by the local loading effect.
In a preferred embodiment, the first pattern has a numerical aperture equal to or greater than 70%.
If the chip pattern or first pattern has a numerical aperture equal to or greater than 70%, namely, has a darkness rate equal to or smaller than 30%, a difference in a numerical aperture between the area C and the central portion 41b becomes remarkable, resulting in reduction in a pattern accuracy which reduction is caused by the local loading effect. Hence, the first pattern is designed to have a numerical aperture equal to or greater than 70%.
In another aspect of the present invention, there is provided a method of fabricating a photomask, including the steps of (a) forming a light-impermeable film on a photomask substrate, (b) applying a resist onto the light-impermeable film, (c) exposing the resist to a light in a pattern comprised of a first pattern corresponding to a circuit pattern of a semiconductor integrated circuit, and a second pattern formed around the first pattern for adjusting a numerical aperture of the first pattern, (d) developing the resist to define a resist pattern, and (e) etching the light-impermeable film with the resist pattern being used as a mask.
In accordance with the method, by adjusting a numerical aperture of the second pattern, it would be impossible to increase a numerical aperture of the above-mentioned area C, and thereby equalize a numerical aperture of the area C to a numerical aperture of the above-mentioned central portion 41b. Hence, by balancing numerical apertures of the area C and the central portion 41b to such a degree that the chip pattern is not adversely affected by local loading effect, it would be possible to avoid reduction in an accuracy with which a resist is etched for forming a pattern, and have a mask pattern or a resist pattern with high accuracy in accordance with a circuit pattern.
In a preferred embodiment, the method further includes the steps of (f) fabricating at least two design data used for designing the second pattern, the design data having different data rates from one another, (g) calculating an average data rate of the first pattern, and (h) selecting design data which has a data rate closest to the average data rate, the resist being exposed to a light to define the second pattern, based on the data rate having been selected in the step (h).
In accordance with the embodiment, data for designing the second pattern is determined before determining data for designing the first pattern. Hence, a photomask could be rapidly designed. In addition, since data for designing the second pattern is determined for every data rate, and design data having a data rate closest to the average data rate is selected, it would be possible to effectively design the second pattern.
Herein, a data rate indicates a ratio of a light-impermeable pattern to the entirety of a photomask. A data rate is coincident with a darkness rate, and is contrary to a numerical aperture.
In a preferred embodiment, the method further includes the steps of (f) dividing an area in which the second pattern is to be formed, into grids, (g) fabricating at least two design data used for designing the second pattern, among first design data and second design data, the first design data designating grids equally spaced away from one another in both a first direction and a second direction perpendicular to the first direction, a pattern being formed in accordance with the first design data, the second design data having a data rate of 0%, (h) calculating an average data rate of the first pattern, and (i) selecting design data which has a data rate closest to the average data rate, the resist being exposed to a light to define the second pattern, based on the data rate having been selected in the step (i).
In accordance with the embodiment, data for designing the second pattern is determined before determining data for designing the first pattern. Hence, a photomask could be rapidly designed. In addition, since data for designing the second pattern is determined for every data rate, and design data having a data rate closest to the average data rate is selected, it would be possible to effectively design the second pattern.
Since an area in which the second pattern is to be formed is divided into grids, and a pattern is formed in accordance with the first design data, it would be possible to form the second pattern having a uniform data rate or numerical aperture. Furthermore, by varying spaces between the grids in the first and second directions, the second pattern having different data rates could be formed. Since the second pattern has a uniform data rate, the second pattern uniformly carried out its performance, ensuring that reduction in a pattern accuracy, caused by the local loading effect, can be uniformly prevented.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.